Dissecting the mechanisms that regulate cell number during the development of the nervous system is of primary importance to understanding normal brain development as well as the aberrant control of cell growth that occurs in malignancies of the CNS. Genetic analysis of cell cycle regulation during Drosophila visual system development has identified the small eye mutant Drop (Dr) as a novel cell cycle control gene required for the initiation of eye morphogenesis. Developmental analysis of the dominant Drop eye imaginal disc phenotype suggests that the dominant alleles interfere with the expression of string (stg), the fly homolog of the yeast cell cycle regulator CDC25. The lack of anterior string expression in the mutant blocks the initiation and progression of the morphogenetic furrow resulting in a failure of photoreceptor cell differentiation leading to subsequent cell death. Starting with a P[LacZ,w+] induced intragenic revertant of DrMio, approximately 65 kb of genomic DNA from the distal 99A region of the third chromosome has been isolated containing part or all of the Drop gene. Analysis of the P-element insertion site revealed that the DrMio chromosome contains a 3S18 retrotransposon not present in two different wild type strains. Insertion of the P-element into the 3S18 retrotransposon resulted in the complete suppression of the dominant small eye phenotype. Genomic Southern blot analysis of a collection of Drop mutations revealed that the dominant gain of function allele L2 and the loss of function alleles suD10, suD21, suD22, and suD61 are associated with deletions that remove part or all of the cloned region. Screening of an eye imaginal disc cDNA library has identified several candidate transcription units within the cloned interval. Experiments are currently in progress to determine: 1) which transcript encodes the Drop gene, 2) its pattern of expression in the eye disc and embryo, 3) the primary structure of the Drop gene product, and 4) the analysis of the loss of function phenotype of Drop with regard to mitotic control during embryonic and imaginal development.